I tested another report with 3 charts. 2 charts are working correctly, one chart is corrupt. It is really strange, I found out that the chart is not bound any longer to a grid (chart properties). The chart type is displayed (Bar chart), but all other information are empty (e.g. Grid, Data Range)

I have no Idea why the connection to the grid has been lost. The grid is still in the report and it's working.